Conventionally, a color printer, color copying machine, color facsimile, and the like have been put into practice as image forming apparatuses for printing color images.
In recent years, a price reduction and image quality improvement of these color image forming apparatuses, and development and proliferation of a peripheral device and computer for inputting image information, and image processing software for editing image information are striking. As a result, an image forming system that combines these apparatuses may easily copy an image of paper money, security, or the like, which should not be printed personally.
To solve such problem, a method of appending information indicating, e.g., the manufacture number of an image forming apparatus or the like to a print image in a format imperceptible to the human eye by the image forming apparatus, and specifying the image forming apparatus by recognizing the manufacture number from the print image later as needed has been proposed.
However, the conventional image forming apparatus appends additional information in consideration of only the image density designated by print image information. That is, information is appended by a single method even when the image density to be formed actually is different from the density designated by print image information due to the individual difference, aging, or the like of the apparatus, or when the print density designated by print image information is different from the image density to be formed actually since the user has designated the print density.
For this reason, the appended information may appear as considerable noise in a print image, or it may become difficult to detect additional information from a print image.